Dust collectors for use in removing various types of particulate matter and impurities entrained in or carried by a flow of transport fluid, such as air, have been known for some time, and when such dust collectors are intended for industrial use they will typically include a plurality of filter elements disposed in a dirty air chamber so that the transport air is caused to flow inwardly through filter elements so as to deposit the particulate matter on the exterior surface of filter elements and permit the cleaned air to be removed from the interior portion of the filter elements and discharged through a clean air chamber in the dust collector. The filter elements are usually fabric bags, as disclosed for example in U.S. Pat. No. 1,743,934, or cylindrically-shaped pleated paper filter elements as disclosed for example in U.S. Pat. Nos. 4,218,227, 4,395,269 and 4,277,260.
When dust collectors of this general type are used in industrial or commercial applications, there is usually a relatively high concentration of particulate matter in the transport air, and, therefore, this particulate matter tends to collect quickly on the exterior surface of the filter elements and prevent the flow of transport air therethrough, whereby the filter element will become incapable of carrying out its intended function unless the collected particles are periodically removed from the exterior surface of the filter element.
The typical method of removing such particles from the filter element is to periodically provide a reverse flow of cleaning air that is introduced at the interior of the filter means and caused to flow outwardly through the filter element in a direction opposite to the normal flow of the transport air, this reverse flow of cleaning air causing at least some of the particulate matter collected on the exterior surface of the filter elements to be moved away from the filter element so that the force of gravity will move the particulate matter downwardly, and ultimately reach the bottom portion of this dirty air chamber where they are collected in a hopper for periodic removal or, for example, they are sometimes removed continuously through an outlet by a vacuum or through a rotary valve. This reverse flow of cleaning air may take a variety of forms, but the most common form is to create at the interior of the filter element a brief, high pressure pulse of cleaning air that pulses each filter at periodic intervals, such as when the pressure drop across such filter element reaches a preset maximum value (e.g. two inches of water) as a result of the particulate matter collecting on the exterior surface of the filter element.
Since the normal inward flow of the transport air through the filter elements is only briefly interrupted by the reverse flow of the cleaning, many of the particles that are moved outwardly away from the surface of the filter element become re-entrained in the transport air when it resumes its normal flow through the filter element, and these re-entrained particles will be carried back to the surface of the filter element and again collected thereon. However, since such particles will be caused by gravity to move downwardly at least a short distance each time they are removed from and then brought back to the surface of the filter, they will gradually migrate downwardly along the surface of the filter element until they eventually reach the bottom portion thereof and fall by gravity to the bottom portion of the housing. While this gradual migration of at least some of the particles results in continuous ultimate removal of the particles from the surface of the filter elements, the relatively slow rate of such removal and the continuous re-entrainment of particles in the transport air imposes a heavy filtering load on the filter elements and thereby limits significantly the so-called air-to-cloth ratios (e.g. the ratio of the volume of transport air to the area of the filtering media presented by the filter elements). For example, in most industrial applications of dust collectors which utilize pleated cellulose paper filter elements having a high number of pleats per inch, the air-to-cloth ratio must be maintained at or below 3, and in many cases it cannot exceed 2. This limitation is quite significant because as the air-to-cloth ratio becomes smaller for any given application, the filtering equipment that must be provided for a given volume of transport air must be increased, thus increasing the capital and operating costs associated with the equipment as well as the floor space that must be dedicated to support the filtering equipment.
Additionally, it is economically desirable to position the filter elements in a dust collector as close to one another as possible so that a given size of dust collector will include as much filtering surface area as possible as discussed for example in U.S. Pat. No. 4,655,799. However, when the filter elements are located closely adjacent to one another, the problem of re-entrainment is exacerbated by the cross-contamination of some particles which are removed from the surface of one filter element and carried to the surface of an adjacent filter element. One proposal for dealing with the problem of cross-contamination of adjacent filter elements is disclosed in U.S. Pat. No. 4,775,398, wherein the generally square-shaped clean air chamber is provided with a plurality of partitions that divide the clean air chamber into four smaller chambers so that one filter element can be located in each of the smaller chambers to isolate each filter element therein and thereby prevent cross-contamination. While this arrangement does deal to some extent with the problem of cross-contamination, it appears to require a somewhat increased spacing between adjacent filter elements so as to increase the size of the dust collector unit, and, more importantly, it does not reduce re-entrainment problems at the surface of each individual filter element and may actually exacerbate this re-entrainment problem by requiring a closely confined chamber for each filter element that can cause more particles removed from such filter element to be re-entrained in the transport air flowing therethrough.
Finally, there are commercially available dust collectors that include a generally cylindrical chamber in which a plurality of vertically extending bag filters are located, each bag being provided with its own cleaning jet. The transport air is introduced into the chamber tangentially with respect to the curved inner wall thereof so that a helical flow path is established. In these dust collectors, a small portion of the particles removed form the exterior surfaces of the bags immediately adjacent the helical flow of the transport air may be reintroduced into such flow, but there is no process for dealing with re-entrainment at other portions of the filter bags, and there is no provision for dealing with cross-contamination of adjacent filter bags.
In accordance with the present invention, a dust collector is provided which significantly reduces both types of re-entrainment discussed above, and which increases significantly the air-to-cloth ratio relative to conventional dust collectors of this general type.